A variety of different types of devices are used to enable a user to communicate with a computer. Perhaps the most common devices are the keyboard and the computer mouse. However, in recent years, touchpad sensing technologies have gained in popularity. These touchpads allow a user to control the cursor or select an option on a computer screen by touching the touchpad with their finger. There several different designs for these touchpads. One of the most popular designs is based on capacitive touchpad technology. The basic principle behind the operation of capacitive touchpad technology is the electrical phenomenon referred to as capacitance. Any two electrical conductors separated by an insulator, such as air, form a capacitor. The two conductors interact with each other by means of their electric fields and for a given potential difference, or voltage between the two conductors, a certain amount of electric charge will be present at the conductor surfaces nearest each other. One conductor will have a net positive charge, and the other conductor will have a net negative charge of equal magnitude but opposite sign. For a given physical arrangement of the two conductors, and fixed properties for the insulating material, the relationship between the voltage between the conductors and the net charge on the conductors is a constant called the capacitance. The capacitance is not dependent on the voltage between the conductors or on the charge, but rather gives the relationship between the two for a given system geometry and insulating material. This relationship can be expressed equation form as:Q=C*V  (1)where Q is the magnitude of charge on each conductor in Coulombs, V is the voltage between the conductors in Volts, and C is the capacitance of the two-conductor system in Farads. When two conductors are oriented such that the distance between them is relatively constant over the surface area of the conductors, and this distance is very small when compared to the area of the conductors, the capacitance can be approximated by the formula:C=∈*A/d  (2)Here C is the capacitance in Farads, d is the separation between the conductors in meters, A is the area of the conductors in square meters, and ∈, the dielectric constant, is a constant associated with the properties of the insulating medium between the conductors, and has units of Farads per meter.
Capacitive touchpads work due to the highly conductive nature of the human body. The fundamental structure of touchpads involves an x-y grid of wires on the top two layers of a printed circuit board. A mixed signal ASIC on the bottom side of the board along with other electronics interface to the grid of wires and perform computations to determine finger placement and pressure. A thin insulator, typically Mylar, is placed over the top surface of the grid to provide the dielectric for the capacitance as well as mechanical protection and isolation. When there is no contact with the touchpad, the nearest conductor to the electrodes is relatively far away, and the capacitance measured at each electrode is thus very low. When a human finger is placed on the pad surface, there is now a higher capacitance present between the finger and the nearby electrodes, due to the conductive nature of the finger and the close proximity of the finger to the pad (decrease in d from equation 2). This increase in capacitance at the nearest electrodes is detected by the touchpad control circuitry and used to calculate finger position, and also finger pressure on the touchpad. As a user presses harder on the touchpad surface, more of the finger's surface area is placed directly over the grid electrodes resulting in a larger capacitance, due to an increase in A from equation 2, which is interpreted by the touchpad electronics as a higher pressure reading.
The touchpad controller measures the capacitance at each electrode position by sending a periodic AC current signal at a fixed frequency and amplitude to each electrode and then, during each cycle interval, measuring the charging time of the resulting AC voltage signals. These voltage signals are small in amplitude, typically no more than a few hundred millivolts, and are similar to sinusoids in that they have 50% duty cycle. The relationship between the current “through” and voltage across a capacitor as seen in equation form is:I=C*dV/dt or I*dt=C*dV  (3)
Thus for a given charging current, the time required to charge a capacitance to an incremental change in voltage is directly proportional to the magnitude of that capacitance. Each cycle, the touchpad electronics send a known AC current to each electrode capacitance and then measure the time required for the voltage across each electrode to increase to a certain threshold voltage. When the capacitance at an electrode is increased this time interval will increase. The touchpad ASIC detects this change and interprets the capacitance increase as the presence of a finger at the appropriate touchpad coordinates.
Unfortunately, capacitive touchpad do not work well with pens or stylus because these devices do not have a capacitance that is comparable to a human finger. Thus, touchpads are typically designed for use with either a pen-like object or the finger of an individual, but not both.